The invention relates to industrial alkaline water electrolysis, i.e. to electrolysis of alkaline electrolytes such as caustic soda or caustic potash, aimed at the production of highly pure hydrogen gas in large scale.
Water electrolysis cells known in the art can be grouped into two families according to the separator used, namely cells equipped with gas-tight ion-exchange membranes and cells equipped with porous diaphragms.
Membrane cells normally permit the production of high purity hydrogen by virtue of the gas-tight properties of the membrane, acting as a hydraulically impermeable wall; oxygen can thus penetrate to the cathodic compartment only by solid state diffusion across the polymer structure. On the other hand, some serious drawbacks are associated with the use of ion-exchange membranes, which are affected by a significant ohmic drop especially at the high current density regime which is required by mass production of hydrogen. The resistive penalty is not only due to the intrinsic properties of the material, which relies on ion conduction in a polymer matrix rather than in aqueous phase, but also to the need of equipping such membranes with an internal reinforcement, such as a PTFE web, for ensuring the required reliability in terms of mechanical properties and dimensional stability for large scale systems. Besides imposing an increase in the thickness of the membrane in which it is embedded, the reinforcement web reduces the polymer section available for ion migration: the unavoidable consequence of these two combined factors is a further increase of ohmic drop. The use of non-reinforced membranes in water electrolysis cells has been described, but the results in terms of reliability of operation proved largely unsatisfactory.
Diaphragm cells known in the art present opposite characteristics and drawbacks. On one hand, the electrolyte which fills the internal porosity of the diaphragm allows to operate with much lower ohmic drops even at very high current densities; on the other hand, the hydrogen purity is severely penalised by the oxygen travelling across the diaphragm. This inconvenience is caused by the diffusion rate of oxygen in the liquid phase filling the porosity, much higher than diffusion rate across the polymer structure of an ion-exchange membrane. In addition, a certain fraction of the diaphragm porosity is usually filled with gas, as the result of the balance between hydrophilicity and hydrophobicity of the diaphragm material. Therefore, the production of low purity hydrogen is an intrinsic characteristic of all porous diaphragm cells.
In an attempt to reduce the cell voltage and increase the purity of product hydrogen in diaphragm cells, Vermeiren et al. disclosed (International Journal of Hydrogen Energy 34(2009), 9305-9315) the use Electrode Diaphragm Electrode assemblies (EDE) obtained by casting as a single unit comprising the anode, the diaphragm and the cathode, wherein the electrode component is either a cast or a non-woven nickel electrode. In this way, a reasonably reduced and stable cell voltage with a fair oxygen purity can be obtained at moderate current densities, e.g. 2 kA/m2; however, stable operation at high current density (such as 4 kA/m2) cannot be attained with this arrangement and hydrogen purity remains still far from what obtainable with a membrane cell. Vermeiren et al. attribute such phenomena to lack of understanding of the relation between operating working conditions and performances, also suggesting the fact that electrode porosity should probably be increased.
There has thus been evidenced the need of providing a water electrolysis cell capable of providing high purity hydrogen at high current density and extremely low voltage, especially for large scale hydrogen production applications.